Heat pump systems have found widespread application for heating and cooling homes and businesses. Because heat pump systems utilize the same primary components for both heating and cooling, they eliminate the need for separate heating and cooling systems and are therefore economical to install and use. Heat pump systems are also highly efficient, resulting in decreased energy costs to the consumer. As a result, the demand for heat pump systems in residential and business applications has continued to grow in recent years.
The use of conventional heat pump systems in colder climates, however, presents significant challenges. In heating mode, a heat pump system draws heat energy from the outdoor air to heat an indoor space. Even at low ambient temperatures, heat may be drawn from the outdoor environment by evaporating refrigerant in an outdoor evaporator. The evaporated refrigerant is then compressed by one or more compressors and then cycled to an indoor condenser where the energy of the compressed refrigerant is released to the indoor space. The refrigerant is then cycled back to the outdoor evaporator to repeat the cycle.
At very low temperatures it becomes increasingly difficult to draw heat from the outdoor environment. In addition, at low temperatures, the outdoor heat exchange coil is very susceptible to frost build up, which limits air flow across the coil. As a result, the performance and efficiency of heat pump systems decreases drastically at very low ambient temperatures when heating capacity is most needed. To address this issue, increased compressor capacity is required for heat pump systems installed in colder climates. Single compressor systems have been utilized that can provide heating at low to moderate ambient temperatures, but such systems typically demonstrate decreased efficiency and performance at higher ambient temperatures relative to systems with less heating capacity. Additionally, such systems must cycle on and off frequently at higher ambient temperatures, resulting in a reduced lifespan for the compressor and decreased system efficiency. Variable speed compressors have been used to address this problem, but these types of compressors are expensive and lead to increased installation costs for the system.
Multiple compressor systems have been proposed to adapt the heat pump concept for use in colder climates. These systems utilize a primary compressor for heating and cooling in moderate temperatures, and also include a booster compressor to provide increased capacity at very low temperatures. An economizer, which utilizes a diverted portion of the refrigerant flow to subcool the refrigerant flowing to the evaporator, may also be used to provide increased heating capacity at very cold temperatures. Systems utilizing multiple compressors and an economizer are disclosed, for example, in U.S. Pat. Nos. 5,927,088, 6,276,148 and 6,931,871 issued to Shaw. Although the systems disclosed in these patents address the need to provide increased heating capacity at very cold temperatures, those of skill in the art have continued to seek sophisticated methods that effectively control the multiple compressors to maximize system efficiency and utilize the full output potential of the compressors.
In particular, prior art systems have controlled multiple compressors based on limited system inputs. For example, the '148 and '871 patents issued to Shaw disclose dual compressor systems that select compressor output in response to decreases and/or increases in outdoor ambient temperature. The '871 patent issued to Shaw discloses a system that selects compressor outputs in response to a multi-step indoor thermostat and the system low side pressure, which pressure is commensurate with outdoor ambient air temperature during all heating cycle modes of operation. These control methodologies, however, may lead to frequent calls for changes in compressor output, which will cause one or both of the compressors to cycle on and off. Although important to prevent unsafe and inefficient compressor operation, a control scheme that more effectively manages when compressors are turned on and off is desirable. Such a system may lead to increased compressor run times in a consistent output condition, which increases the life of the compressors and overall system efficiency.
Prior art systems have disclosed the use of multiple compressors to provide heat for an indoor forced air heat exchanger. With multiple compressors, however, additional heating capacity is present that may also be utilized for additional indoor heating systems such as a hydronic floor system. The heat pump system may also provide energy for a tap water heater. With these additional heating components integrated into the heat pump system, the potential output of the compressors may be more fully realized, providing further justification for the cost of the system. Further, if properly configured and controlled, these additional heating components may be used to absorb excess energy produced by the compressors to address and limit high pressure and temperature conditions. Also, with multiple heating components receiving energy input from the compressors, compressor run time can be increased. With the compressors cycling on and off less frequently, the life span and efficiency of the compressors is increased.
Despite the increased capacity provided by multiple compressors, heat pump systems installed in very cold climates may require some form of back up heating to address the very coldest conditions. Prior art systems, however, have not effectively integrated control of the back up heating system with the control of the heat pump system. As a result, the back up heating system, which performs at lower efficiency, is over utilized as compared to the heat pump system, leading to increased energy costs. If the two systems are effectively integrated and controlled, the higher efficiency of the heat pump system may be more fully utilized even during the coldest months of the year.
Finally, those of skill in the art have sought a heat pump system that effectively integrates utility Load Management Control. Load Management Control, or LMC, allows a utility company to remotely and temporarily shut down certain users' heating and cooling systems at times when the utility is experiencing peak loads. Because this capability is desirable for utility companies, energy consumers that implement this feature may receive decreased energy rates, tax incentives or other consideration. To implement LMC, an auxiliary heating system with a different energy source, such as a gas furnace, is typically required to provide heat when the utility initiates a system shut down in cold weather conditions. Control of this alternative heating source is preferably integrated with control of the heat pump system so that the system effectively and efficiently transitions to the alternative heat source when a shut down command is received, and also easily transitions back to the main heating system when the shut down condition terminates.
Accordingly, an object of the present invention is to provide a heat pump system for use in colder climates that is economical to install and use.
An additional object of the present invention is to provide a heat pump system with multiple compressors that effectively controls the compressors to maximize system efficiency and utilize the full output potential of the compressors.
A further object of the present invention is to provide a heat pump system with multiple heat outputs including a forced air heater, a hydronic floor heating system and/or a water heater.
Yet another object of the present invention is to provide a heat pump control system that may easily and effectively divert compressor energy to multiple heat outputs to fully utilize the output of the compressors, address high pressure and temperature conditions, increase compressor run times, decrease compressor cycling and maximize the overall efficiency of the system.
Still another object of the present invention is to provide a heat pump control system that effectively integrates a back up heating system for use in the very coldest conditions.
A still further object of the present invention is to provide a heat pump system that effectively integrates utility Load Management Control.
Additionally, an object of the present invention is to provide a heat pump system that may effectively defrost an outdoor coil.
Finally, an object of the present invention is to provide a heat pump system that provides energy for heating tap water when the system is in use for either heating or cooling, and also minimizes the use of the water heater element under all conditions.